The highly orchestrated muscle activation sequences during motor behaviors are achieved directly through the fine-tuned firing of motor neurons in the ventral spinal cord. These motor neurons are mainly regulated by spinal interneurons present in all mammals, which are, in turn, connected to other spinal neurons as well as various types of descending neurons from the brain including corticospinal (CS) neurons (CSNs). CSNs located in the motor cortex connect to spinal interneurons to control motor neuron activity in all species, and thereby coordinate the activity of flexor and extensor limb muscles to control skilled movements. Although we and others mainly focused on outputs of CSNs through their axons, CSNs also receive inputs from their presynaptic neurons through their dendrites. However, the identification and understanding of the function of presynaptic neurons of CSNs (pre-CSNs) remains limited. We developed rabies virus-based assays to identify pre-CSNs. We hypothesize that each population of pre-CSNs will be distinctly activated to control discrete phases of skilled movements and muscle activation. To test our hypothesis, in Aim 1 we will map presynaptic partners of CSNs in the brain. We will further determine whether those connections are functional (Aim 2). Finally, we will determine how pre-CSNs control forelimb skilled movements and muscle activity (Aim 3). These results will provide the necessary framework for not only defining spinal motor circuitry, but also subsequent development of novel targeted interventions to treat motor disabilities.